Hydrophones have been used for many years for detection and location of ships and submarines and other underwater targets. Hydrophones are also used for sensing sonic waves in underwater geophysical exploration. A transmitting or active hydrophone sends out sonic waves for impingement upon and reflection from an underwater target to determine its location and other information. A receiving or passive hydrophone receives sonic waves from underwater sources such as noise generated by a submarine or sonic waves reflected from an object. A single receiving hydrophone may be used for "listening" for the presence of an underwater object. A combination of transmitting and receiving hydrophones may be used to determine the presence and location of objects by reflection of sonic waves. Hydrophone arrays are used for determining direction, distance and other information about underwater objects.
The transducer in a hydrophone converts such pressure waves to corresponding electrical signals for vice versa, depending upon whether the hydrophone is a receiver or transmitter. The transducer element for producing this conversion is a piezoelectric body which most commonly takes the form of a peizoelectric ceramic material such as lead zirconate titanate or barium titanate. Such ceramic elements are advantageous in that they are highly efficient energy converters and are rugged and can be shaped into suitable configurations. The ceramic element may be used in an associated mechanical structure or it may be configured to serve as the mechanical structure itself as in a bimorph with a pair of ceramic disks bonded to opposite faces of a diaphragm mounted for vibration.
Although piezoelectric ceramic bodies work well as transducers in hydrophones and other applications, the inherent characteristics also produce certain undesirable effects. The high efficiency in converting sonic pressure waves to electrical signals results from the fact that when they are stimulated in a vibratory mode they exhibit a high ratio of stored energy to dissipated energy. This characteristic in an analogous electrical circuit is referred to as "high Q". In the equivalent electrical circuit, the high Q is achieved with inductive and capacitive reactances which are larger compared to the resistance of the circuit. As a result of the high Q, piezoelectric transducers exhibit sharply defined resonance characteristics when exposed to variable frequency sound waves or when stimulated by variable frequency excitation voltage. In other words, the transducer responds strongly at its natural resonant frequency but has a much weaker response to higher and lower frequencies. This can be a serious disadvantage in sonic sensing or sonic generation where a broad spectrum of sound frequencies is to be monitored or generated.
The resonant amplitude response, referred to above, is accompanied by a rapidly changing phase-versus-frequency characteristic. This is an undesirable characteristic especially in an array of hydrophones used to determine direction. In such an application, a fixed or linearly varying phase shift as a function of frequency is desired.
The high Q characteristic of the ceramic transducer also creates a transient response problem when the transducer is driven by an impulsive type of signal, i.e. a signal with an amplitude which increases and decreases very quickly. The transducer responds slowly at the beginning of the signal and continues to respond after the signal input has been terminated. This phenomenon, commonly called "ringing", results in distortion of the output signal relative to the input signal. The poor transient response reduces the intelligibility of received signals and produces errors in distance measurement when hydrophones are used in navigation or sonar systems.
Another problem with hydrophones utilizing ceramic transducers is that of variations in the sensitivity of the transducer resulting from manufacturing variables. This can be overcome by individual calibration but such a procedure is difficult and costly.
In the prior art, attempts have been made to overcome the problems with piezoelectric ceramic transducers; however, such attempts have met with limited success or have resulted in complex and expensive transducers. A common procedure in the prior art is to reduce the electromechanical Q of the transducer by either mechanical or electrical damping. Either method reduces the efficiency of the transducer and results in the need for additional signal amplification and often reduces the signal-to-noise ratio. Also, the amount of damping must be individually matched to the transducer to accommodate variations in transducer characteristics. This procedure is time consuming and expensive.
Also, in the prior art, it has been attempted to reduce the above-mentioned problems by the use of negative feedback in the signal amplifier used with the hydrophone. Negative feedback from the output of the amplifier to the input of the amplifier does not correct for undesired hydrophone characteristics. When a separate hydrophone is included in the feedback loop, the system becomes unstable due to nonlinear transport lag and phase shift and from the mismatch between the two transducers.
An objective of this invention is to overcome certain disadvantages of the prior art piezoelectric ceramic transducers.